This invention relates to rosin ester based hot melt adhesives and, more specifically, to the stabilization of such adhesives with zinc salts.
Rosin is the common resin of several varieties of the pine tree. It is mainly a mixture of seven different twenty-carbon monocarboxylic acids and other minor components. The rosins to which this invention relates include gum rosin, wood rosin, and tall oil rosin or the rosin acids contained therein, such as abietic acid.
It is possible to esterify rosin; the resulting rosin esters have numerous applications, including many in the adhesives industry. For example, rosin esters impart wetting and hot tack to commercial hot melt adhesives; ethylene-vinyl acetate (EVA) based hot melt adhesives often contain up to 65% rosin ester as a tackifier. Rosin esters are also used in solvent-based and water-based adhesive systems.
Several factors generally govern the choice of a rosin ester in an EVA-based hot melt adhesive system. Two major factors are the compatibility of the rosin ester with other components of the system and the effect the rosin ester has on the viscosity stability of the system.
The compatibility of a given rosin ester with other components in hot melt adhesives depends upon a number of factors relating to the process and reagents used in making the ester. Clarity is a desired property for hot melt adhesives; good compatibility is indicated when a hot melt adhesive is clear upon formulation and remains free of haze during thermal aging.
It is also desirable that the viscosity of a hot melt adhesive remain stable for a commercially reasonable period after it is formulated. Viscosity stability, like compatibility, is affected by the process and reagents used to synthesize the rosin ester tackifier, with a commercially significant variation of viscosity over time indicating poor viscosity stability.
Although the particular process and reagents used to synthesize a given rosin ester intimately affect both compatibility and viscosity stability for a hot melt adhesive, those conditions which contribute to good compatibility are not necessarily the same as those which contribute to good viscosity stability. On the contrary, in many cases the conditions which lead to acceptable viscosity stability result in poor compatibility. For example, the esterification of many rosin esters is catalyzed by mineral acid esters and by mineral acids such as phosphorus oxy-acids. Mineral acid esters can be transformed into mineral acids and can thus serve as latent acid catalysts. Rosin esters synthesized with both types of acid catalysts often have excellent compatibility with a wide range of EVA-based hot melt adhesives if the resultant residual acidity in the product is not neutralized. However, the viscosity stability of such un-neutralized systems is usually very poor due to acid-catalyzed degradation of the EVA. Viscosity stability may be achieved for such systems by reducing their acidity. One method for reducing acidity is to treat rosin esters with alkali metal hydroxides. U.S. Pat. No. 4,548,746, Duncan, et al., for example, teaches the use of sodium hydroxide in this regard. However, the hot melt adhesives incorporating esters so neutralized are usually hazy, either upon formulation or upon thermal aging, indicating poor compatibility. Some improvement over sodium hydroxide is seen by neutralization with calcium acetate, but serious hazing problems still remain. Organic amines also neutralize the ester acidity, but impart dark color and haze to hot melt adhesive systems.
Although U.S. Pat. Nos. 168,253 and 4,284,543, each in the name of Hollis, teach the use of a lithium salt to extend molten stability in hot melt adhesive compositions, lithium salts generally impart darker color and higher levels of haze than is commercially desirable.
U.S. Pat. No. 4,725,384, Du Vernet, teaches a process for esterifying rosin with a polyhydric alcohol in the presence of a catalytic amount of phosphinic acid. Magnesium salts of organic acid are used in this process to neutralize the acidity contained therein. Rosin esters synthesized in this manner are said to improve the clarity and viscosity stability of EVA-based hot melt adhesives into which they are incorporated as tackifiers. The patent specifically teaches the use of magnesium acetate to neutralize the acidity of an ester so synthesized. Such improved rosin esters provide, to corresponding hot melt adhesive formulations, improvement in both compatibility and viscosity stability. Du Vernet also discloses the inoperability of the corresponding calcium salt for this purpose.
It is still desired to provide improved rosin ester compositions comprising rosins esterified with polyhydric alcohols wherein the resulting compositions may be added to hot melt adhesives and other materials to provide such adhesives having a diminished tendency towards hazing and improved viscosity stability.
Accordingly, it is an object of this invention to provide methods for the production of rosin esters with substantial or total neutralization of any acid catalyst employed in their synthesis.
A further object is to provide rosin esters which impart to hot melt adhesives good tackification without significant hazing or instabilities in viscosity.
A further object of the invention is to provide improved rosin esters which have good utility as tackifiers and otherwise and which have all or part of their acidic components neutralized. Yet another object is to improve the physical and chemical properties of rosin esters and improved tackifiers for hot melt and other adhesives produced therefrom.
These and other objects of the invention will become apparent to persons of ordinary skill in the art from a review of the present specification and appended claims.